Theoretical And Experimental Research Of The High Power And High Energy Fiber Lasers In MID-IR

In recent years, the high power and high energy mid-infrared fiber lasers operating around 3 μm have attracted large amount of attention because of their wide applications in medical, materials processing, atmospheric monitoring, defence, scientific reserch etc. Moreover, the high power and high energy fiber lasers combine a series of merits of optical fiber laser such as high beam quality, convenient intergration, excellent heat dissipation, high flexibility, and the characteristics of the water absorption of mid-infrared laser. It has been taken as one of the main developing directions of fiber laser in the future. In this dissertation, our study mainly focuses on the high power CW fiber laser and the high energy pulse fiber laser at 3μm. The main reseach achievements are shown follows:Firstly, a numerical model involving rate equations, power evolution and heat dissipation equtions capable of describing the cascaded transitions of Er3+-doped ZBLAN fiber laser and Ho3+-doped fiber laser is presented. The performance of the cascade laser was tested against fiber lengths, dopant concentration, energy transfer process, output coupling, pump schemes and heat generation. The results suggest that the slope efficiencies and thresholds for both transitions increase with increasing Ho3+ or Er3+ concentration with the slope efficiency stabilizing after 1 mol% rare earth doping. Decreasing the output coupling for the upper(～3 μm) transition decreases the threshold of the lower transition and the upper transition benefits from decreasing the output coupling for the lower transition for both cascade systems. The highest slope efficiency was achieved under counter-propagating pump conditions. Saturation of the output power occurs at comparatively higher pump power with dilute doping compared with heavier doping. Overall, we show that the cascade lower concentration fluoride laser is the best candidate for high power output because of its higher slope efficiency and lower temperature excursion of the core and no saturation of the output.Secondly, based on the above theoretical optimization results, we set a high power cascade Er3+-doped ZBLAN fiber laser with a 975 nm high power laser as the pump source. A maximun output power of 15.2 W at the wavelength of 2885 nm was achieved with a slope efficiency of 26.7% when the pump power was increased to 64 W. This is the first 10 watt-level fiber laser at 3μm waveband without any special active or passiev cooling designs based on the diluently Er3+-doped ZBLAN fiber. Meanwhile, we observed the cascade laser at 1610 nm at the launched pump power of 17.28 W. Its maximum output power was 3.2 W with a slope efficiency of 7.1%.Thirdly, an 1150 nm diode-pump passively Q-switched Ho3+-doped ZBLAN fiber laser using topological insulator(TI): Bi2Te3 as the saturable absorber(SA) was acheved. A series of characterizations were performed to exhibit the great performance of the TI: Bi2Te3. Inserting this component into a linear cavity Ho3+-doped ZBLAN fiber laser, stable Q-switched pulses at 2979.9 nm were obtained with the repetition rate of 81.96 kHz and pulse duration of 1.37 μs. The achieved maximum output power and pulse energy were 327.4 mW at a slope efficiency of 11.6 % and 3.99 μJ respectively, only limited by the available pump power. Our work reveals that the TIs are absolutely a class of promising and reliable SAs for pulse generation at 3 μm mid-infrared waveband.Lastly, a high energy Er3+-doped ZBLAN fiber laser based on the MOPA system was demonstated where a passively Q-switched pulsed fiber laser with a designed SESAM(Semiconductor Saturable Absorber Mirror) as the saturable absorber was employed as the seed laser. The repetition rate was 107.5 kHz and the pulse width was 1.29 μs. Then, the seed laser was coupling into the Er3+-doped ZBLAN fiber amplifier. An amplified pulse laser with a a repetition rate from 94.3 kHz to 151.5 kHz and a pulse width from 0.82 μs to 1.29 μs at a pump power of 4.46 W was achieved. Its wavelength ranged from 2820 nm to 2835 nm due to the accumulated populations on 4I13/2 level as increased pump power up-shifted the terminating Stark manifold of 4I11/2â†'4I13/2 transition. As increasing the pump power, the pulse laser became distortion until disappearing as a result of the self-exciting oscillation.